Transcutaneous monitoring of blood gases: is it comparable with arterialized earlobe sampling?

Researchers are increasingly looking for reliable non-invasive methods of assessing blood gas concentrations, and several new techniques have recently become available. Values derived using arterialized earlobe samples have been found to be comparable with conventional arterial samples and recent studies have compared transcutaneous blood gas analysis with the traditional arterial samples and found a reasonable level of agreement in particular for the partial pressure of carbon dioxide. There are no data comparing oxygen and carbon dioxide partial pressures (pO(2), pCO(2)) derived from arterialized samples with one of the newer transcutaneous techniques. We therefore simultaneously studied arterialized earlobe blood gas samples and values for pO(2) and pCO(2) obtained by a transcutaneous monitor (TINA, Radiometer, Copenhagen) in 26 subjects with varying blood gas values. There was a close agreement between the two methods for assessment of pCO(2) [mean difference (95% C.I.) between transcutaneous and earlobe values 0.25 kPa (-0.004, 0.5 kPa)], but not for pO(2) [1.71 kPa (0.35, 3.07 kPa)]. Similarly, the limits of agreement were narrow for pCO(2) compared to those for pO(2) (-0.98, 1.47 kPa and -6.44, 3.02 kPa respectively). We conclude that transcutaneous measurement of pCO(2) using the TINA is acceptable in the research setting, whereas assessment of pO(2) cannot reliably be made using this technique.